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Abstract: Life extension of the existing nuclear power plants imposes significant challenges to core structural materials that suffer increased fluences. This paper presents the microstructural evolution of a type 304 stainless steel and its variants alloyed with extra Ni and Cr under neutron irradiation at ∼320°C for up to 10.2dpa. Similar to the reported data of type 304 variants, a large amount of Frank loops, ultrafine G-phase/M23C6 particles, and limited amount of cavities were observed in the irradiated samples. The irradiation promoted the growth of pre-existing M23C6 at grain boundaries and resulted in some phase transformation to CrC in the alloy with both extra Ni and Cr. A new type of ultrafine precipitates, possibly (Ti,Cr)N, was observed in all the samples, and its amount was increased by the irradiation. Additionally, α-ferrite was observed in the type 304 steel but not in the Ni or Ni+Cr alloyed variants. The effect of Ni and Cr alloying on the microstructural evolution is discussed. [Copyright &y& Elsevier]

Abstract: Zirconium or hafnium additions to austenitic stainless steels caused a reduction in grain boundary Cr depletion after proton irradiations for up to 3dpa at 400°C and 1dpa at 500°C. The predictions of a radiation-induced segregation (RIS) model were also consistent with experiments in showing greater effectiveness of Zr relative to Hf due to a larger binding energy. However, the experiments showed that the effectiveness of the solute additions disappeared above 3dpa at 400°C and above 1dpa at 500°C. The loss of solute effectiveness with increasing dose is attributed to a reduction in the amount of oversized solute from the matrix due to growth of carbide precipitates. Atom probe tomography measurements indicated a reduction in amount of oversized solute in solution as a function of irradiation dose. The observations were supported by diffusion analysis suggesting that significant solute diffusion by the vacancy flux to precipitate surfaces occurs on the time scales of proton irradiations. With a decrease in available solute in solution, improved agreement between the predictions of the RIS model and measurements were consistent with the solute-vacancy trapping process, as the mechanism for enhanced recombination and suppression of RIS. [Copyright &y& Elsevier]

Abstract: The deformation microstructure of proton-irradiated stainless steels may play a key role in explaining their irradiation-assisted stress corrosion cracking (IASCC) susceptibility. In the present study, three model alloys (UHP-304, 304+Si, 304+Cr+Ni) with different stacking fault energies (SFEs) were irradiated with 3.2MeV protons at 360°C to 1.0 and 5.5dpa and then strained in 288°C Ar atmosphere. The deformation microstructure of the strained samples was investigated using scanning electron microscopy and transmission electron microscopy. The results showed that the slip lines interacted with grain boundaries by grain-to-grain transmission, grain boundary sliding or deformation ledge formation at grain boundaries. Expanded channels, which were formed at locations where dislocation channels intersected the grain boundaries or other channels, were found predominately in the low SFE alloys UHP-304 and 304+Si. The steps and shear strain at grain boundaries caused by channel expansion may increase the IASCC susceptibility in low SFE stainless steels by producing strain concentrations and inducing cracks in the oxide film. [Copyright &y& Elsevier]

Abstract: Grain boundary engineering (GBE), primarily implemented by thermomechanical processing, is an effective and economical method of enhancing the properties of polycrystalline materials. Among the factors affecting grain boundary character distribution, literature data showed definitive effect of grain size and texture. GBE is more effective for austenitic stainless steels and Ni-base alloys compared to other structural materials of nuclear reactors, such as refractory metals, ferritic and ferritic–martensitic steels, and Zr alloys. GBE has shown beneficial effects on improving the strength, creep strength, and resistance to stress corrosion cracking and oxidation of austenitic stainless steels and Ni-base alloys. [Copyright &y& Elsevier]

Abstract: The microstructural evolution, specifically of grain boundaries, precipitates, and dislocations in thermomechanically processed (TMP) Alloy 800H samples was characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The TMP not only significantly increased the fraction of low-Σ coincidence site lattice boundaries, but also introduced nanoscale precipitates in the matrix and altered the distribution of dislocations. Statistical analysis indicates that the morphology and distribution of grain boundary precipitates were dependent on grain boundary types. The microstructure optimization played a synergistic effect on the significantly increased strength with comparable ductility and enhanced intergranular corrosion resistance and creep-fatigue life compared to the as-received samples. [Copyright &y& Elsevier]